amptech question: tubing a fireball - please help

[hoehlentroll]

I read that the "grading" systems makes retubing a piece of cake. I opened the amp and found this:

I bought the amp new in local store and the tubes where stock. the tubes are about 5 years old :Smokedev: *i know, I know*

1. what does the different tubing do to the sound?
2. could this by any means be NO fault?
3. what should I do now? a. buying grade 22 or 27 or b. take the amp to a tech which means LOADS of money. I live near Tube Amp Doctor (TAD-tubes).

I have no fucking idea of how to bias a tube amp!

EDIT: funny thing: http://www.urbandictionary.com/define.php?term=tubing

 

[TheWinterSnow]

The grading number is simply the amount of idle current that the tube will conduct when the tube is placed in a typical amp. Higher numbers means that the tube will run a little warmer. You should pick up a matched set, meaning that they are the same or one number off, just buy a set of matched tubes, put em in and be done with it.